Cassanda was a tragic figure in Greek mythology who was given the ability to see the future, but cursed so that no one would ever believe her. So she would see all the horrors that would come, but be completely unable to prevent them. George is in a similar situation, since as long as he's being held captive, he can't warn anyone about Helmut's upcoming attack.

Of course, the real reason the Helmeted Author doesn't kill George is that he needs George to convince Nate to remove his helmet later. Presumably there is something preventing Helmut from removing it himself, so he needs to manipulate others to do it for him, and this is all part of the plan.